Preparation Of Controllable Reduced Graphene Oxide And Graphene-based Nanocomposite And Their Application As Substrates For SERS

Surface-enhanced Raman scattering （SERS） is an surface alternativeenhancement technique, it will increase the raman intensities of molecules adsorbed ata certain substrate up to several orders of magnitude, and it greatly promotes the application oframan spectroscopy used for scientific researching and characterizing the structure ofmaterials. For SERS, though there are numerous experimental and theoretical workson it, there are also many controversies about the mechanism and no complete pictureof the enhancement mechanism is available even now. Normally, the two widelyaccepted mechanisms are electromagnetic mechanism （EM） and chemical mechanism（CM）. EM is mainly due to the surface plasmons excited by the incident light, andwas achieced some results now. However, the origin and the role of the chemicalenhancement are constantly under debate because of its enhancement with theelectromagnetic enhancement. It is, therefore, important to find a substrate that doesnot possess the electromagnetic enhancement, retaining the chemical enhancementalone and independently tunable.Graphene, has the ideal2D structure with a monolayer of carbon atoms packedinto a honeycomb crystal plane, is a special material with many spark stellarperformance. Considering graphene, first, its surface is relatively smooth in despite offluctuations that follow from the under lying substrate. Second, the opticaltransmission through the graphene surface in the visible range is higher and is morethan95%. Besides, the surface plasmon on graphene is in the range of terahertz ratherthan in the visible range. So，on the basis of these considerations, graphene does notsupport EM. On the other hand, for CM graphene has possibilities. This paper wasprepared graphene by Chemical methods, the different reduced degrees of grapheneoxide used as substrates to study its SERS performance, and we analyzed the role ofGO in GO/Au nanocomposites when it used for SERS substrate.In the first part, we synthesized the graphite oxide by the Hummer method,through ultrasound, hydrothermal method was used to reduced graphene oxide（GO）. Respectively, under acidic condition and alkaline condition, graphene with differentreduction degrees was obtained by adjusting the reduction temperatures and times. Itis found that the reduction rate of the graphene oxide under acidic condition is slowerthan the alkaline conditions, and the removal of oxygen functional groups aredifferent at this two systems. Rhodamine6G （R6G） with intense fluorescent effectused for probe molecule, and we used GO and reduced GO as substrates for Ramanenhancement experiments, found that the Raman signal intensity became weak withthe reduction degree improving, and the removal of oxygen functional groups weredifferent, but Raman scattering peak positions did not change, indicating that theinteraction between the SERS substrate and the probe molecule have not connectionwith the types of oxygen functional groups in GO, whereas it only impacted by thedegree of the reduction GO, and restore the more thorough, the weaker signalintensities. In addition, reduced GO can inhibit the fluorescence background of theprobe molecules, indicating that reduced GO has fluorescence quenching effect.The noble metal nanomaterials are commonly used as SERS substrate, they haveexcellent physical and chemical properties, and have strong Raman enhancementactivities. As we know, the Raman scattering signal intensity of molecular depends onthe number of molecules in the enhanced region, but metal surface is difficutlt toadsorb the molecules with benzene ring structure, which limits the Ramanenhancement effect. In the traditional SERS system, metal substrates contact with theprobe molecules directly, there must be exist interactions between both, which willmake the fluorescence background of moleculars to change and difficult to getthisintrinsic Raman scattering signal. In order to obtain an SERS substrate, whichinclude high enhanced activity, perfect fluorescence quenching effect and its surfacehas a strong adsorption for different molecules, therefore, we synthesized GO and Aunanoparticles. The graphene-based nanocomposite used as SERS substrate, andadopted Rhodamine6G (R6G,8×10^-5M/L) and2-mercaptopyridine (2-MPy,1×10^-4M/L) as probe molecules. The study found that: R6G molecules unable adsorbedon the surface of gold nanoparticles, but Raman signals appear after added into theGO and content of GO influences the molecular fluorescence background greatly. 2-MPy as a probe molecule, the Raman scattering intensity is virtually unchanged andonly suppress the fluorescence signal after added into the GO, indicateing that in thegraphene-based nanocomposite substrate, GO plays as a bridge to connect betweenthe metal surface and the probe molecules, and it has fluorescent quenching effect.